This invention relates to epoxy resin-based electrical encapsulation compositions.
Epoxy resins are used to encapsulate electrical components such as integrated circuits. The optimum properties for encapsulation resins include high glass transition temperature in the cured state, crack resistance under soldering, and low chlorine ion generation during cure.
Recent developments in the use and packaging of semiconductive elements include surface mounting of such elements. In the surface mounting process, the semiconductive device is immersed in a bath of molten solder and exposed to high temperature. Under these conditions, any moisture in the encapsulating material will expand and crack the encapsulating material.
The currently most widely used epoxy-based encapsulating formulation contains an o-cresol novolac epoxy resin, a phenol novolac curing agent, and triphenylphosphine cure accelerator. Such a formulation has a relatively low glass transition temperature and an unacceptably high tendency to crack during soldering.
Another problem which must be addressed in high-performance electrical encapsulation is corrosion of aluminum wiring by moisture and chloride ions. It is therefore important that encapsulation formulations have good moisture resistance and low chlorine ion generation during cure. These properties are to some extent dependent on the cure accelerator used. For example, triphenylphosphine reduces chlorine generation but is a less active curing accelerator, producing a cured material having a low glass transition temperature. On the other hand, imidazole cure accelerators tend to provide cured products having higher glass transition temperatures but an increased level of chloride ion generation.
The properties of the encapsulating material are also dependent on the epoxy resin used. Bishydroxybiphenyl-type resins are used for encapsulation, but these resins suffer from low glass transition temperature compared to EOCN-type resins. When the bishydroxybiphenyl-type resins are used, it is therefore of even greater importance to use a cure accelerator which maximizes cured glass transition temperature and minimizes chloride ion generation.
It is therefore an object of the invention to provide an epoxy-based electrical encapsulation formulation which has a high cured glass transition temperature, crack resistance under soldering and low chloride ion generation.